Stable and effective proton exchange membrane formation via cross-linking the polymeric proton donor and proton acceptor in a layer-by-layer structure

Systematic arrangement of the proton donor and acceptor as a layer-by-layer (LbL) structure is one of the simplest surface modification methods to improve the proton conductivity of a proton exchange membrane. In general, LbL assembly occurs via physical or chemical interaction between each deposited layer. Here, the stabilization of the proton donor and acceptor polymers by cross-linking those two layers together was applied as a concept. The challenge was the molecular design of the proton donor and accepter polymer species to each have compatible cross-linkable functional groups as well as proton transfer species in the molecules. Poly(acrylic acid) decorated with different amounts of thiol groups (PAA-M) was synthesized, while benzimidazole decorated branching polyethyleneimine was functionalized with maleimide groups (BIm-PEI-MI). The cross-linkable proton donor and acceptor were alternately LbL deposited on a sulfonated poly(ether ether ketone) (SPEEK) surface. The LbL membrane with an optimum crosslink level showed a proton conductivity of up to 2 orders of magnitude higher than that of the pure SPEEK membrane even at a temperature as high as 170 °C.